I believe the perfect battle rifle cartridge was developed over 100 years ago. The 6.5 Swedish Mauser. There is a point where all the ballistic compromises meet. The 6.5 is powerful without excessive recoil.
I actually think the AR-15/.223 combo is about perfect for use against people.
I once fired a bullpup based on a Remington fireball action. There is something very unsettling about having that explosion going off right beside your face.
The case on the 6.5X55 is a little long. Considering that the 7.62 (.308) is on a 51mm case, and the 5.56 is 45mm, you could probably shorten the 6.5 down to 48 to 50mm length and still have excellent ballistics. The 6.5 Swede is definately a sweet round though.